One of the strategies to increase the power density of supercapacitors is the optimization of the interface between active material and current collectors, electrodes, or electrochemical interfaces, which are referred to herein as electrode surfaces. Herein a current collector is a thin film that is electrically connected to a larger conductive body to promote interaction between the conductive body and an active material, the conductive body and current collector forming an electrode. An electrode or electrochemical interface may be karstified without any obviously separate or separable current collector. As far as the contact impedance between the active materials and the electrode surfaces can be reduced, improvements can be obtained. It is well known in the art that the ability to collect electrical charge from active materials is a function of surface area of the electrode surface. It has been suggested to increase a surface area of current collectors using nanoporous substrates (see Chang, J. K., S. H. Hsu, et al. (2008) “A novel electrochemical process to prepare a high porosity manganese oxide electrode with promising pseudo capacitive performance” Journal of Power Sources 177(2): 676-680 and Yoon, Y. I., K. M. Kim, et al. (2008) “Effect of nickel foam current collector on the supercapacitive properties of cobalt oxide electrode” Journal of the Korean Ceramic Society 45(6): 368-373). Other methods for surface treatment of metallic electrode surfaces include growing porous carbon films and carbon nanotube brushes on various metallic current collectors to serve as substrates for deposition of active material. Such nano-architectured electrodes may be expected to outperform untreated electrode surfaces, but they are more expensive.
It is known in the art to use chemical etching to remove oxides and develop pitting corrosion, and then applying a conductive carbonaceous material, for example by sol-gel (see Portet, C., P. L. Taberna, et al. (2006) “Modification of Al current collector/active material interface for power improvement of electrochemical capacitor electrodes” Journal of the Electrochemical Society 153(4): A649-53), and to apply a conformal carbon layer onto porous aluminum by a chemical vapour deposition, where the deposition itself removes an oxide layer, replacing it with an interfacial layer of Al4C3 (see Hsien-Chang, W., L. Yen-Po, et al. (2009) “High-performance carbon-based supercapacitors using Al current-collector with conformal carbon coating” Materials Chemistry and Physics 117(1): 294-300). Activated carbon-based supercapacitors with carbon-coated aluminum current collectors have exhibited remarkable performance.
Conductive zinc films have been deposited on the surface of Al foils to improve performance of the foils as current collectors (see Zhang, B.-h., G.-x. Zhang, et al. (2007) “Influence of modified current collector on double layer capacitor” Chinese Journal of Power Sources 31 (7): 538-41). These results also show that the modified current collectors can significantly reduce the resistance between current collector and electrode active materials and improve the utilization of electrode active materials.
The desire for high surface area current collectors is equally useful for other capacitor electrodes, and electrochemical cells, for example, as noted by WO 00/19465 to Jerabek et al. Specifically Jerabek et al. notes that surface etching and other roughening procedures can be used to enlarge the contact area, but warns that the permanence of a treated current collector is an issue in device longevity since the electrode surface can become chemically transformed, as with an oxide or can react with electrode or electrolyte to form a barrier. Jerabek et al. teaches coating a solid, nonporous, current collector, nominally of aluminum but could also be copper or steel, with a protective coating consisting of a metal nitride, boride or carbide, to prevent an oxide from developing at the interface with the electrode active materials. The coating of the collector can be made by reactive sputter deposition in a vacuum, conventional sputter deposition, evaporation, reactive evaporation, molecular beam deposition processes, and any of a host of other plasma or energy-enhanced deposition processes in vacuum.
Similarly, EP 2525377 to Yuriy et al. appears to teach use of ion bombardment or plasma for removing an oxide layer and to roughen an Al foil. Translated from French by Google translate, Yuriy teaches: [0009] “The use of on cannon or high frequency plasma generator allows: to eliminate the native oxide and eventual pollution of the surface of the aluminum foil film, and increase primary roughness of this surface,” An abstract of Yuriy et a from corresponding WO 20121156809 indicates that a method for manufacturing a current collector for a supercapacitor comprises processing a surface of an aluminium foil in a vacuum chamber, including removing a native oxide film and applying a current-conducting coating, said current-conducting coating comprising an outer layer consisting of carbon deposited by sputtering a powder mix of carbon and aluminium onto the aluminium foil.
Aluminum is one of the most common materials for electrode surfaces used in energy storage devices such as supercapacitors and Li-ion batteries (as well as other electrochemical cells) due to its light weight, low cost and high electrical conductivity, however, aluminum oxidizes very quickly in air, and particularly in aqueous electrolyte solutions or in organic electrolyte with water impurity, to form an insulating layer that is typically about a few nanometer thick, known as a native oxide film. There is further concern, as noted in Jerabek et al. that in operation, especially if an aqueous electrolyte is used, the oxidation layer may continue to grow and decrease performance of the aqueous electrolyte based supercapacitor.
Unfortunately, the chemical etching techniques available today to pattern metallic foils are problematic in many ways. There are environmental issues with chemical etching, and reproducibility is also an issue. Importantly, residual etch, reactants, and byproducts are of concern for long term cyclic stability of the interface between the electrode surface and electrode active materials. Plasma or ion gun etching suggested by Yuriy et al. will effectively remove a native oxide film in preparation for deposition of a current-conducting carbon coating, but this etching will not substantially increase a surface area of the surface, unless a high spatial focus is applied to the beam (which runs counter to the purpose of removing the native oxide film), and the beam dwells on etch points an inordinately long duration (at least several hours) to achieve a few micrometer etching depth. Chemical etchants typically remove layers with limited ability to control surface quality/roughness and so they do not tend to produce deep and controllable etching patterns. Etching chemicals, temperature and time are key influential factors on surface finish quality during chemical etching. Therefore a need remains for producing deep patterns with controlled morphology into foil surfaces; producing the deep patterns with high spatial variation in depth, like in naturally karstified structures (referred to herein as karstification and its word forms). Karstified morphology is denotes patterns that result in higher surface areas than conventional surface roughening, and etching can produce. Metal foils with deep patterns and controlled morphology can be used as current collectors, especially if the electrode active material has a form that matches the foil surface morphology allowing for intimate contact of the electrode active material over a great surface area. The form naturally depends on particle size and shape, as well as distributions thereof.
There is further a need for thin film coatings to protect aluminum current collectors, electrodes, etc. from surface oxidation and the resultant formation of electrical insulation at the electrode active material—current collector interface. The coatings need to be electrically conductive, nonreactive with active materials, and corrosion resistant in the intended electrochemical environment. The need is particularly important for electrochemical cells (supercapacitors and batteries) using aqueous electrolyte or corrosive salts in organic electrolytes.
In an unrelated field of technology, pulsed laser deposition has been developed to produce an Al coating onto a desired surface in a high vacuum by ablating a target, such as a thick block of Al. It had previously been observed by Applicant that pulsed laser deposition creates highly irregular surfaces on the target (i.e. the Al block). There has been little report of this, but Stephen R. Foltyn (“Surface modification of materials by cumulative laser irradiation”, Chapter 4, of the book entitled “Pulsed Laser Deposition of Thin Film”, edit by Douglas B. Chrisey and Graham K. Hubler, A Wiley-Interscience publication, 1994) does teach the generation of laser cones in laser irradiating single-crystal Al2O3 at 266 nm, and indicates quite generally that “By the early 1980s ripple patterns, now known as laser-induced periodic surface structures (LIPSS), had been produced in metals, semiconductors, and dielectrics”. The reference shows ceramic cones that have diameters at base around 10-20 μm and a height of at least twice that. The reference does not teach that such structures can be produced on metallic foils or a use as an electrochemical interface, or current collector.